1. Field of the Invention
This invention relates in general to a feedthrough fastener and a method for forming same for fiberoptic cable systems containing buffered fibers and more particularly to a feedthrough fastener and a method for forming same utilizing an injection mold wherein the buffered fibers are not subject to stress during or after the injection molding process and wherein the injection mold allows the integrity and support of the strength member of the cable system to be maintained in the feedthrough fastener.
2. Description of the Prior Art
Fiberoptic cables are quickly displacing copper wiring as the medium of choice in local area network systems. The fiber environment, however, raises certain unique problems resulting from the delicate nature of fibers. For example, fiberoptic systems are extremely susceptible to signal loss or distortion, and even permanent damage to the fiber, when excessive bending of the fiber occurs or when forces such as pulling or crushing are exerted on the fiber.
These problems have been addressed by the placement of fibers within a cable system comprised of an outer jacket, which in turn surrounds a strength member comprised of aramid fibers, such as kevlar, which in turn surrounds one or more buffered fibers. This cable system allows stress from pulling and crushing to be absorbed by the outer jacket and strength member. In addition, these components provide a certain degree of rigidity to the cable, thereby protecting against extreme bending. Even with these features, however, fiberoptic cables must still be handled with care.
The delicate nature of fibers is a factor that comes into play when the end of a fiber is terminated or spliced. A certain length of the outer jacket and strength member is often stripped from the buffered fiber at the termination or splice point. The remaining length of exposed buffered fiber is generally spliced to another section of exposed buffered fiber or attached to a fiberoptic connector. The section of exposed buffered fiber, however, is highly susceptible to damage from external forces.
Some protection is afforded to the section of exposed buffered fiber by enclosing this section in a housing such as a wallbox. Such housings are well known and examples are disclosed in U.S. Pat. Nos. 4,874,904 and 4,850,901. While these housings protect the exposed buffered fiber from direct forces, the risk still exists of damage to the exposed buffered fiber from forces exerted on remote sections of the cable system.
For example, a pulling force exerted on a remote section of the cable system may be transmitted axially along said cable system and be absorbed by the exposed buffered fiber at its splice/connection point. This force can seriously damage the fiber since it must absorb this force without the aid of the strength member of the cable system. A pulling force may also be exerted in a direction that results in a sharp bend in the cable system at the point it enters the housing, i.e., about the edge of the aperture in said housing. Such a bend, even if at a point where the cover and strength member still surround the buffered fibers, can damage the fiber if it is severe enough.
These problems are well known and a number of prior art methods and devices have been used to attempt to resolve these problems. Each of these methods or devices, however, has disadvantages and fails to adequately protect the integrity of the buffered fibers.
In particular, it has generally been recognized that it is desirable to maintain the protection afforded by the cable system's strength member in order to protect the integrity of the buffered fibers. For example, one method that is used is to secure the cable system's strength member to the housing. This is done, for example, by tying an exposed length of the aramid fibers to a portion of the housing. This method generally fails to maintain the same degree of protection afforded by the strength member to the cable system since it doesn't eliminate stress on the buffered fibers when external forces are imposed. Furthermore, it fails to address the damage caused to the buffered fibers if severe bending occurs about the aperture to the housing.
Other attempts to resolve these problems have utilized various devices wherein compression pressure on the cable system is utilized to hold it in place within the housing and as some protection against remote forces. For example, cable ties--as disclosed in U.S. Pat. No. 4,875,881, grommets--as disclosed in U.S. Pat. No. 4,717,231, and clamps--as disclosed in U.S. Pat. No. 4,812,004 have been used to secure cable systems to housings. The use of each of these devices has two primary disadvantages.
First, regardless of how tightly each of these devices hold the cable system, there is no direct connection between the device and the cable system's strength member. Accordingly, some degree of the protection afforded by the strength member is lost. Second, inherent in the use of a compression force to secure the cable system in place is some risk of damaging the buffered fibers and their signal paths as a result of a strong compression force. More likely, however, this compression force will eliminate the free movement of the buffered fibers within the cable system; thereby causing the jacket, strength member and buffered fibers to act as one element. When this occurs, a remote force exerted on the cable system that would otherwise be absorbed by the jacket and strength member will also be absorbed by the buffered fibers, resulting in damage to the fibers.
A relatively quick and inexpensive method of securing the cable system to the housing would be to utilize an injection mold shaped to conform to the aperture in the housing and formed about the cable system. The use of an injection mold for this purpose has been limited, however, inasmuch as the injection molding process may exert a substantial amount of stress on those elements within the interior of the mold.